A Study of Vacuum Requirements for the Acceleration of Heavy Ions in an MP Tandem Van De Graaff

Accelerator vacuum systems, which are adequate for the acceleration of intense beams of light elements, may not be suitable for heavy ions which have large charge changing cross sections. Charge charging collisions with residual gases in the accelerator tubes produce a continuous distribution of ion energies as well as the desired discrete energy output of the accelerator. An MP tandem with a carbon foil terminal stripper was used to accelerate Iodine ions. The attenuation of the unique energy beam components due to unwanted charge changing collisions was measured by energy and magnetic analysis of the accelerator output. The effects of terminal pumping, vacuum deterioration due to added gas, and reduction of the accelerator tube conductance were studied with particular attention to high charge states. Under typical conditions with a 6 MV terminal and pressure ~ 4 ?? 10-6 Torr in the terminal region, the total beam attenuation was ~20%.